C h a p t e r 3 4
Organization and Control of Neural Function
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The Autonomic Nervous System
The autonomic nervous system (ANS), in contrast to the
previously discussed somatic nervous system, provides a
person with the ability to maintain internal physiologic
homeostasis and perform the activities of daily living in
an ever-changing physical environment. The term “auto-
nomic” (self-governing) reflects the independent nature
of this part of the nervous system or functioning largely
below the level of consciousness. The ANS is involved in
regulating, adjusting, and coordinating vital visceral func-
tions such as heart rate and blood pressure. It is strongly
affected by emotional influences and is involved in many
of the expressive aspects of behavior, including blushing,
pallor, palpitations, clammy hands, and dry mouth.
As with the somatic nervous system, the ANS is rep-
resented in both the CNS and the PNS. Traditionally,
the ANS has been defined as a general efferent system
innervating visceral organs. The efferent outflow from
the ANS has two divisions: the sympathetic nervous sys-
tem and the parasympathetic nervous system. The affer-
ent input to the ANS is provided by visceral afferent
neurons, usually not considered part of the ANS.
The functions of the sympathetic nervous system
include maintaining body temperature, respiration,
digestion, elimination, and adjusting blood flow and
blood pressure to meet the changing needs of the body.
The sympathoadrenal system also can discharge as a unit
when there is a critical threat to the integrity of the indi-
vidual—the so-called fight-or-flight response. During a
stress situation, the heart rate accelerates, the blood pres-
sure rises, blood flow shifts from the skin and gastrointes-
tinal tract to the skeletal muscles and brain, blood sugar
increases, the bronchioles and pupils dilate, the sphinc-
ters of the stomach and intestine and the internal sphinc-
ter of the urethra constrict, and the rate of secretion of
exocrine glands that are involved in digestion diminishes.
Emergency situations often require vasoconstriction and
shunting of blood away from the skin and into the mus-
cles and brain, a mechanism that, should a wound occur,
would provide for a reduction in blood flow and preser-
vation of vital functions needed for survival.
In contrast to the sympathetic nervous system, the
functions of the parasympathetic nervous system are
concerned with conservation of energy, resource replen-
ishment and storage, and maintenance of organ function
during periods of minimal activity. The parasympathetic
nervous system slows heart rate, stimulates gastrointes-
tinal function and related glandular secretion, promotes
bowel and bladder elimination, and contracts the pupil,
protecting the retina from excessive light during periods
when visual function is not vital to survival.
The two divisions of the ANS usually are viewed as
having opposite and antagonistic actions (i.e., if one acti-
vates, the other inhibits a function). Exceptions are func-
tions, such as sweating and regulation of arteriolar blood
vessel diameter, that are controlled by a single division
of the ANS, in this case the sympathetic nervous system.
The sympathetic and parasympathetic nervous sys-
tems are continually active. The effect of this continual
or basal (baseline) activity is referred to as
tone
. The
tone of an effector organ or system can be increased
or decreased and usually is regulated by a single divi-
sion of the ANS. For example, vascular smooth muscle
tone is controlled by the sympathetic nervous system.
Increased sympathetic activity produces local vasocon-
striction from increased vascular smooth muscle tone,
carry efferent axons from motor neurons located
within the ventral horn of the gray matter. At its
distal end, the ventral root joins with the dorsal
root to form a mixed spinal nerve.
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The brain can be divided into three regions: the
hindbrain, the midbrain, and the forebrain.The
hindbrain, consisting of the medulla oblongata,
pons, and cerebellum, contains the neuronal
circuits for the eating, breathing, and locomotive
functions required for survival and cranial
nerves V through XII.The midbrain contains
cranial nerves IV and III.The forebrain consists
of the diencephalon, which forms the core of the
forebrain; and the telencephalon, which forms
the cerebral hemispheres.
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The diencephalon contains the thalamus and
hypothalamus. All sensory pathways have direct
projections to the thalamic nuclei, which convey
the information to restricted parts of the sensory
cortex.The hypothalamus functions in the
homeostatic control of the internal environment.
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The cerebral hemispheres, which are the lateral
outgrowths of the diencephalon, are divided into
four lobes—the frontal, parietal, temporal, and
occipital lobes.The premotor area and primary
motor cortex are located in the frontal lobe; the
primary sensory cortex and somatosensory
association area are in the parietal cortex;
the primary auditory cortex and the auditory
association area are in the temporal lobe; and
the primary visual cortex and association visual
cortex are in the occipital lobe.
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The brain is enclosed and protected by
connective tissue sheaths called the meninges,
which consist of three layers: the dura mater,
arachnoid mater, and pia mater.The CSF, in which
the brain and spinal cord float, is secreted into
the ventricles by the choroid plexus, circulates
through the ventricular system, passes outside
to surround the brain, and is reabsorbed into the
venous system through the arachnoid villi.The
blood-brain barrier and CSF-brain barrier protect
the brain from substances in the blood that
would disrupt brain function.
